1. Field of the Invention
The present invention relates to a substrate for reflection type liquid crystal display elements, and more particularly to a substrate for a reflection type liquid crystal display element that partially transmits light.
2. Related Background Art
In recent years, with an object of making portable electronic devices thinner and lighter, and also making prolonged operation with a battery as a power source possible, reflection type liquid crystal display elements that make use of external light and are characterized by having low power consumption have started to be used. One such reflection type liquid crystal display element that has been proposed is a semi-transmitting type liquid crystal display element that uses a semi-transmitting plate as a reflector plate, and can be used both as a reflection type liquid crystal display element in bright locations, and as a transmission type liquid crystal display element using a backlight in dark locations (see Japanese Laid-open Patent Publication (Kokai) No. 11-002709).
The liquid crystal display element substrate used in this semi-transmitting type liquid crystal display element has a constitution in which a liquid crystal layer is held between a pair of transparent substrates provided with liquid crystal driving electrodes, and the light scattering of the liquid crystal layer is controlled by means of the magnitude of a voltage applied across the liquid crystal layer. Moreover, a plurality of semi-transmitting reflector plates each formed of a thin film of a metal such as aluminum are laminated on the back side substrate. The thin metal film is made sufficiently thin as to partially transmit light.
Moreover, recently a substrate for a reflection type liquid crystal display element having, in place of thin metal films forming the semi-transmitting reflector plates, a multilayer dielectric film comprised of a predetermined number of transparent films laminated on top of one another, with these transparent films being formed of two dielectric materials having different refractive indexes to one another, has been proposed by the assignee of the present application (see Japanese Patent Application No. 2000-204952, which is an application claiming priority based on Japanese Patent Application No. 11-32689).
However, according to the above-mentioned reflection type liquid crystal display element having a multilayer dielectric film, there is a possibility of ripples occurring in the optical reflection spectrum or optical transmission spectrum across the visible region, resulting in a problem in which it is not always possible to stably obtain an optical characteristic of a desired flatness, and coloring due to reflected light occurs.
Moreover, by making the number of layers in the multilayer dielectric film of the order of, for example, a few tens to a few hundreds, the degree of freedom to design the optical characteristic increases, with it being possible to set specifications such that the occurrence of ripples is suppressed, but there is a problem that as the number of layers is increased, the time taken to build up the layers increases to an industrially impractical extent.
It is an object of the present invention to provide a substrate for reflection type liquid crystal display elements having a multilayer dielectric film which can have fewer layers than according to the conventional art and can thus be formed in a shorter time, and which can stably obtain an optical characteristic of a desired flatness across the visible region, and moreover can prevent the occurrence of coloring due to reflection and transmission.
To attain the above object, the present invention provides a substrate for a reflection type liquid crystal display element, comprising a transparent substrate, and a reflective mirror formed on top of the transparent substrate, wherein the reflective mirror comprises a predetermined number of high-refractive-index first transparent films and low-refractive-index second transparent films laminated alternately on the transparent substrate, and wherein either or both of the first transparent films and the second transparent films are arranged such that a film thickness thereof increases progressively or decreases progressively with distance from the transparent substrate.
According to the above constitution, high-refractive-index transparent films and low-refractive-index transparent films are laminated alternately to form a multilayer dielectric film, and as a result the number of layers in the multilayer dielectric film can be made smaller, and hence the multilayer dielectric film can be made in a shorter time, the cost can be reduced, and mass production can be carried out more easily. Moreover, either or both of the high-refractive-index transparent films and the low-refractive-index transparent films are arranged such that the film thickness thereof increases progressively or decreases progressively with distance from the transparent substrate, and as a result an optical characteristic of a desired flatness across the visible region can be obtained stably, and the occurrence of coloring due to reflection can be prevented.
Preferably, the first transparent films are arranged such that a film thickness thereof increases progressively or decreases progressively with distance from the transparent substrate.
Preferably, the predetermined number is in a range of 3 to 14.
Preferably, the predetermined number is 3 or 4.
Preferably, each of the first transparent films has a refractive index of at least 1.8 at a wavelength of 550 nm, and each of the second transparent films is laminated on top of one of the first transparent films and has a refractive index of not more than 1.5 at the wavelength of 550nm. As a result, the efficiency with which light is used can be improved, and moreover, since thin metal films are not used in any of the transparent films, with regard to the signals inputted into the transparent conductive films, the possibility of signal delay being caused can be eliminated.
Preferably, the first transparent films are formed of a high-refractive-index material having titanium dioxide as a principal component, and the second transparent films are formed of a low-refractive-index material having silicon dioxide as a principal component. As a result, the difference in refractive index between the high-refractive-index transparent films and the low-refractive-index transparent films can be increased, and an optical characteristic of a desired flatness can be obtained with greater certainty.
Preferably, the substrate for a reflection type liquid crystal display element further comprises a base film having silicon dioxide as a principal component laminated on top of the transparent substrate. As a result, the adhesion between the transparent substrate and the transparent film closest to the transparent substrate is improved, impurities leaching out of the transparent substrate are intercepted, and hence an alkali passivation effect can be obtained.
More preferably, one of the first transparent films furthest from the transparent substrate is a photocatalytically active film having titanium dioxide as a principal component. As a result, dirt can be removed easily from the reflective mirror and hence the reflective mirror can be kept clean, and moreover adhesion to a color filter can be improved.
Preferably, the substrate for a reflection type liquid crystal display element further comprises a hydrophilic thin film having silicon dioxide as a principal component laminated on top of the one of the first transparent films. As a result, the hydrophobicity is mitigated and cleaning with an aqueous cleaning solution can be made effective.
More preferably, the substrate for a reflection type liquid crystal display element further comprises a transparent rugged scattering layer laminated between the transparent substrate and the base film. As a result, reflected light in the substrate is scattered to obtain a uniform intensity of reflection over a specific range of angle of visibility.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.